This invention relates to a process of treating condensate formed by a cooling of raw gas produced from solid fuels by a pressure gasification comprising a treatment with oxygen and water vapor. Carbon dioxide may be used as a further gasifying agent.
The solid fuels which are suitable for pressure gasification include mainly coal, brown coal, and peat. It is basically known to gasify such solid fuels under operating pressures in the range of about 5-150 bars, preferably about 10-80 bars. The pressure gasification can be carried out with particularly good results in the Lurgi gas-producing reactor, details of which are described in Printed German Application No. 1,021,116, German Patent No. 2,352,900, and U.S. Pat. Nos. 3,540,867; 3,902,872; and 3,930,811, the disclosure of which are incorporated herein by reference.
The known pressure gasification process results in a raw gas, which leaves the pressure reactor at temperatures in the range of about 400.degree.-700.degree. C. and which contains on a dry basis about 20-80% water vapor and about 1-10 g finegrained solids per standard cubic meter. Tests have shown that the particle size range of the solids entrained by the raw gas depends highly on the space velocity in the reactor. It has been found that the gasification of caking coals and particularly also of swelling coal results in a discharge of more solids than the gasification of non-caking and non-swelling coals. The solids consist mainly of unreacted fuel and of ash particles.
The raw gas discharged from the pressure reactor is first scrubbed in direct contact with water and cooled condensate. This scrubbing results in a removal of solids and in a condensation of part of the hydrocarbons, tars and water vapor. When the gasification results in a raw gas having a high solids content, pumpable condensates may no longer be formed after a short time of operation. It is also known that the processing of caking and swelling coals can be much improved in that the mixture which is obtained after the cooling of the raw gas and which contains tars, hydrocarbons, and solids, is recycled at least in part to the pressure gasification process.
At the present state of the art relating to the pressure gasification of solid fuels, the pressure gasification reactors can be operated at a high throughput rate of about 10-20 metric tons of fuel per hour. This results in an increase of the absolute quantities of solids entrained by the raw gas and also in a larger average particle diameter of said solids. After a certain time of operation these phenomena may result in disturbances in the conduit and pump system. Experiments in which screens were used to separate the coarse-grained solids from the condensate mixture have not given satisfactory results because these coarse separators must be cleaned at a considerable expenditure. The coarse particles which are thus removed still contain high boiling tars and must be destroyed at a considerable expenditure because discarding of such solids is not permissible for ecological reasons.
For this reason it is an object of the invention so to improve a process of the kind defined first hereinbefore that the disadvantages involved in previous proposals are avoided and a trouble-free and economical operation is ensured. This is accomplished according to the invention in that the condensate, which contains tar, solids, and water, is fed to a separator, a heavy fraction having a high solid content is withdrawn from said separator, at least part of said heavy fraction is processed in a solids disintegrator, part of the processed solids having a particle size substantially below about 2 mm are added to the condensate derived from the raw gas produced by pressure gasification, and another part of the processed solids is recycled to the separator.
After the processing in the solids disintegrator, the high-solids fraction derived from the condensate mixture no longer contains any substantial quantity of coarse particles having a size above 2 mm. The processing in the solids disintegrator results also in a mixing of the particles in the condensate to a certain degree and in a homogenization of the mixture. It has been found that the processed mixture no longer tends to form disturbing deposits, particularly in conduits and pumps.
The condensate which is recycled to the pressure gasification process and the condensate which is fed to the separator are suitably kept at a temperature above about 50.degree. C., preferably above about 70.degree. C. The increased temperature prevents solidification of the tar constituents in the condensate. Such solidified tar constituents could promote adhesion of deposits and clogging. The high-solids heavy fraction withdrawn from the separator may have a solids content of about 10-60%. This mixed fraction can be handled without trouble even when its solids content is close to or at said upper limit.
The processing of the condensate in the solids disintegrator is suitably controlled so that a predominant part of the solids in the processed condensate has a particle size below about 0.5 mm. Solids having a small particle size can be more easily maintained in a state of fine division in the tar-containing condensate so that they will not give rise to any trouble.
An embodiment of the process according to the invention will be explained with reference to the drawing which is a schematic flow sheet.
Granular solid fuel, particularly coal, is fed to a pressure gasification reactor 1 through a conduit 2. The reactor 1 is fed with oxygen-containing gas through conduit 3 and with water vapor through conduit 4. The ash left after the gasification is withdrawn in conduit 5. During the pressure gasification in the reactor 1, the fuel is held in a packed bed and moves opposite to the direction of flow of the gasifying agents oxygen and water vapor. The raw gas produced by the gasification flows at temperatures of about 400.degree.-700.degree. C. from the reactor 1 into a scrubber-cooler 7 and is directly contacted therein with a mixture of water and condensate at temperatures of about 160.degree.-220.degree. C. Scrubbed gas which is saturated with water vapor is conducted in conduit 8 to a waste-heat boiler 9. Through conduit 10, part of the condensate collected in the scrubber-cooler 7 is added to the water-heat boiler. The gas which has been cooled in the waste-heat boiler leaves the latter through conduit 11. Condensate collected in the sump of the waste-heat boiler is returned through conduit 12 to the scrubber-cooler 7. Fresh water is added through conduit 13. In some embodiments, the gas having a saturation temperature of 160.degree.-180.degree. C. must be purified further and used, e.g., in a combined gas-steam turbine process. In such case, the waste-heat boiler may be eliminated and the saturated gas leaving the scrubber-cooler is fed to any purification stages which may be required.
A condensate mixture which contains the solids that have been removed from the raw gas in the scrubber-cooler 7 is discharged in conduit 15 and is pressure-relieved in atmospheric pressure in one or more pressure-relief stages. The pressure-relieved mixture is fed to a separator 17, in which the several constitutents of the mixture are separated by gravity. A major portion of the solids mainly together with heavy, high-boiling tar, is collected in the lowermost part of the separator 17. Hydrocarbons, lower-boiling tars and oil are enriched in an intermediate portion above said solids; these constituents contain only little dust and are fed through conduit 18 to means for their further processing. The upper part of the gravity separator 17 contains a mixture which comprises aqueous condensate, light hydrocarbons and light oils. This mixture is withdrawn through withdrawal conduit 19. Pressure-relieved gas leaves the separator through conduit 20.
The high-solids heavy fraction separated in the separator 17 is fed from the lower end of the separator through conduit 21 to a disintegrator 22. The solids are embedded in a vehicle of high viscosity and are disintegrated in the disintegrator 22 to a final particle size upper limit of about 2 mm. The disintegration of the solids results also in a homogenization of the mixture as it passes through the disintegrator. Suitable disintegrators are, e.g., dispersing machines known for the production of pharmaceuticals (Ullmann's Encyklopadie der technischen Chemie, Vol. 4 (1953), pages 21/22). At least part of the solids-containing condensate discharged from the disintegrator 22 is fed by pump 23 through conduit 24 and by a metering pump 25 through conduit 26 and is thus recycled into the upper portion of the pressure gasification reactor 1. The surplus of the condensate mixture is recycled through conduit 27 to the separator 17 and is fed thereto either through a conduit 28, which includes a valve 29, or through a conduit 30, which includes a valve 31. In case the feed of recycled tar-dust mixture via pump 25 and conduit 26 has to be reduced or even stopped, still a certain amount of flow in the system has to be maintained to avoid separation of solids in the mixture. For this purpose conduit 27 is foreseen in which the flow can be adjusted by valve 29 or 31. If the flow in conduit 26 is completely stopped, the amount of flow in conduit 27 should be about 20-60% of the amount of mixture normally withdrawn from separator 17 by conduit 21. Under normal operation conditions the flow rate in conduit 27 should be at least 10% of the normal feed rate in conduit 26 and may be increased in order to adjust the ratio of dust and tar in the mixture withdrawn from separator 17 by conduit 21. Though feeding part of the mixture back to separator 17 via conduit 27 may be advantageous, it is not an essential feature of the process.